The present invention relates to progressive multifocal ophthalmic lenses. Such lenses are well-known; they are suitable for correcting presbytic spectacle wearers, and consequently supply an optical power which is different between near vision and far vision, when mounted in a frame.
Progressive ophthalmic lenses conventionally comprise a far vision region, a near vision region, an intermediate vision region, and a main meridian of progression passing through these three regions. French Patent Application 2,699,294 to which reference can be made for more details, discusses, in its introduction, the various elements of a progressive multifocal ophthalmic lens, together with the work carried out by the applicant with an aim to improving comfort of wearers of such lenses. Briefly, the top portion of the lens is called the far vision region and is used by the spectacle wearer for distance vision. The lower portion of the lens is the near vision region which the spectacle wearer uses for close work, for example for reading. The region extending between these two latter regions is called the intermediate vision region.
In practice, progressive multifocal lenses frequently comprise an aspherical face, and a face which is spherical or toric, which is machined to adapt the lens to the wearer's prescription. It is consequently usual to characterize a progressive multifocal lens by surface parameters of the aspherical surface, specifically a mean sphere S and a cylinder, at each point thereof.
Mean sphere S is defined by the following formula: ##EQU1##
in which R.sub.1 and R.sub.2 our minimum and maximum radii of curvature, expressed in meters, and n is the refractive index of the lens material.
Cylinder is given, using the same conventions, by the formula: ##EQU2##
We now call power addition the difference in mean sphere between a reference point in the far vision region and a reference point in the near vision region. These two reference points are usually chosen to be on the main meridian of progression.
The main meridian of progression is a line which is generally defined as being the intersection of the aspherical surface of the lens and the wearer's glance when the latter looks straight ahead, at various distances. The main meridian of progression is frequently an umbilical line, in other words one for which all points have zero cylinder.
The applicant has also proposed, in order to better satisfy the visual requirements of presbytic spectacle wearers and improve the comfort of progressive multifocal lenses, to adapt the shape of the main meridian of progression, as a function of power addition, and in this respect see French Patent Application 2,683,642.
Existing progressive multifocal lenses can be further improved, notably those having a high power addition. For such lenses, the values of cylinder reach high levels in view of the increase in lens power. This leads to disturbances to dynamic vision and a reduction in the width of the intermediate vision region and close vision region. This is all the more disturbing when one considers that, for prescriptions of power addition greater than 2.50, the wearer no longer disposes of objective accommodation. In such cases, it is consequently better to provide the spectacle wearer with the power addition he or she needs for sharp vision in close vision together with wide and accessible visual fields for near and intermediate vision.
Advantageously, the near vision region is also sufficiently high up to ensure the wearer enjoys optimal comfort.
In French Patent applications 2,683,642 and 2,683,643, the applicant has already proposed improvements consisting in varying the shape of the meridian as a function of power addition and, consequently, the age of the wearer. Lateral offset, at the nasal side, of the close vision reference point, takes account of the moving closer of the reading plane as the wearer's age advances.
Applicant has also proposed to vary the position of the close vision reference point not only as a function of power addition, but also as a function of ametropy, to take account of prismatic effects.
In French Patent application 2,753,805, applicant has disclosed another improvement for determining the meridian. A method employing ray tracing makes it possible to determine the meridian, by taking account of the moving closer of the reading plane as well as prismatic effects. Thus, for a given power addition, wearers suffering from different degrees of ametropia will perceive the same variations in power from the far vision region to the near vision region. Sphere and cylinder management ensure ample fields of vision.